A collision with an asteroid might have set the planet Mercury whirling oddly in its orbit, a new study suggests.

Scientists had long assumed that Mercury was tidally locked with the sun — the planet’s tiny size and proximity to the sun suggested the star’s gravitational pull would quickly force Mercury into such a state. However, radar observations of Mercury surprisingly revealed that the planet led a far stranger life, rotating three times on its axis for every two orbits it completes around the sun. Now, researchers suggest that Mercury was once tidally locked, initially spinning in the opposite direction to its orbit.

“Mercury once had a spin rate synchronous with the sun, like the moon with the Earth,” study co-author Alexandre Correia, a planetary scientist at the University of Aveiro in Portugal, told SPACE.com.

Computer models suggest that a giant impact from an asteroid then knocked it into its current strange configuration. Evidence of this collision might include Caloris Basin, Mercury’s largest impact crater, which matches the predicted size, age and location of the impact, the researchers said. “It is the perfect candidate,” Correia said.

Such an impact might also explain certain hollows seen on Mercury’s surface. If the planet was tidally locked, one side would have been extremely bright and hot while the other extremely dark and cold. Substantial deposits of ice might have accumulated on the dark half, some of which might have been buried under matter ejected from impacts. When Mercury’s spin later changed and daylight began falling on the once dark side, this buried ice might have vaporized, leaving behind hollows, the researchers explained.

The results of the study were published online today (Dec. 11) in the journal Nature Geoscience.